GE N60 Instruction Manual page 431

CHAPTER 5: SETTINGS
The neutral directional overcurrent element provides both forward and reverse fault direction indications for the
and
DIR OC1 FWD NEUTRAL DIR OC1 REV
operating current is above a pickup level (overcurrent unit) and the fault direction is seen as forward or reverse,
respectively (directional unit).
The overcurrent unit responds to the magnitude of a fundamental frequency phasor of either the neutral current
calculated from the phase currents or the ground current. There are separate pickup settings for the forward-looking and
reverse-looking functions. If set to use the calculated 3I_0, the element applies a positive-sequence restraint for better
performance: a small user-programmable portion of the positive-sequence current magnitude is subtracted from the
zero-sequence current magnitude when forming the operating quantity.
The positive-sequence restraint allows for more sensitive settings by counterbalancing spurious zero-sequence currents
resulting from
• System unbalances under heavy load conditions
• Transformation errors of current transformers (CTs) during double-line and three-phase faults
• Switch-off transients during double-line and three-phase faults
The positive-sequence restraint must be considered when testing for pickup accuracy and response time (multiple of
pickup). The operating quantity depends on the way the test currents are injected into the relay (single-phase injection:
I = (1 – K) × I ; three-phase pure zero-sequence injection: I
op injected
The positive-sequence restraint is removed for low currents. If the positive-sequence current is below 0.8 pu, the restraint is
removed by changing the constant K to zero. This facilitates better response to high-resistance faults when the unbalance
is very small and there is no danger of excessive CT errors as the current is low.
The directional unit uses the zero-sequence current (I_0) or ground current (IG) for fault direction discrimination and can
be programmed to use either zero-sequence voltage ("Calculated V0" or "Measured VX"), ground current (IG), or both for
polarizing. The zero-sequence current (I_0) must be greater than the
setting value and IG must be greater than 0.05 pu to be validated as the operating quantity for directional
CUT-OFF LEVEL
current. The following tables define the neutral directional overcurrent element. V_0 is the zero-sequence voltage, I_0 is
the zero-sequence current, ECA is the element characteristic angle, and IG is the ground current.
N60 NETWORK STABILITY AND SYNCHROPHASOR MEASUREMENT SYSTEM – INSTRUCTION MANUAL
NEUTRAL DIR OC1

OFFSET: 0.00 Ω
NEUTRAL DIR OC1 FWD

ECA: 75° Lag
NEUTRAL DIR OC1 FWD

LIMIT ANGLE: 90°
NEUTRAL DIR OC1 FWD

PICKUP: 0.050 pu
NEUTRAL DIR OC1 REV

LIMIT ANGLE: 90°
NEUTRAL DIR OC1 REV

PICKUP: 0.050 pu
NEUTRAL DIR OC1 BLK:

Off
NEUTRAL DIR OC1

TARGET: Self-reset
NEUTRAL DIR OC1

EVENTS: Disabled
operands, respectively. The output operand is asserted if the magnitude of the
I = 3 x (|I_0| - K x |I_1|)
op
Range: 0.00 to 250.00 Ω in steps of 0.01
Range: –90 to 90° in steps of 1
Range: 40 to 90° in steps of 1
Range: 0.006 to 30.000 pu in steps of 0.001
Range: 40 to 90° in steps of 1
Range: 0.006 to 30.000 pu in steps of 0.001
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
= 3 × I .
op injected
PRODUCT SETUP  DISPLAY PROPERTIES  CURRENT
GROUPED ELEMENTS
NEUTRAL
5
Eq. 5-20
5-237